In recent years, increases in communication traffic continue to accelerate, and the development of dense wavelength division multiplexing (DWDM) systems as a high-capacity transmission technology has been advancing rapidly. Although channels with 10 Gbps per wavelength have been multiplexed and transmitted with DWDM systems in the past, the adoption of ultra-high speed transmission formats is advancing, because by adopting ultra-high speed transmission formats having 40 Gbps or 100 Gbps per wavelength, it is possible to reduce the number of line cards and realize lower costs, space savings, and power savings. With these ultra-high speed transmission formats, reductions in OSNR sensitivity, wavelength dispersion, polarization mode dispersion, and the like may occur due to the higher speeds, restricting the transmission range. In order to avoid such transmission range restrictions, the adoption of formats such as DQPSK transmission format and the DP-QPSK transmission format, which reduce the baud rate by imposing signal information onto the optical phase state in addition to the optical intensity, is promising. Receiver circuits used with such transmission formats use an optical circuit called a delay line interferometer (DLI) or a dual polarization optical hybrid (DPOH) to convert the optical phase difference of signal light into an optical intensity difference, and the converted optical intensity is detected with a photodiode (PD). In recent years, there has been sharply rising demand for an integrated receiver module integrating an optical circuit that conducts optical phase-optical intensity conversion on an input optical signal as discussed above, a PD, and a high-frequency amp that includes functionality for current-to-voltage converting a signal converted into an optical current by the PD, and for amplifying and outputting the result as a high-frequency electrical signal. Such a conventional integrated receiver module is disclosed in PTL 1, for example.
FIGS. 1A to 1C and FIGS. 2A to 2C illustrate examples of an integrated receiver module applying an optical circuit, PD, and high-frequency amp as discussed above. FIGS. 1A to 1C and FIGS. 2A to 2C are both an example of an integrated receiver module that receives a 40 Gbps DQPSK optical signal using a DLI 2 fabricated with a planar lightwave circuit, a back-illuminated PIN-PD array 5, and a differential input TIA 6. FIGS. 1A and 2A are side views, FIGS. 1B and 2B are top views, and FIGS. 1C and 2C are front views seen from the plane of optical incidence of the back-illuminated PIN-PD array 5.
The configuration in the integrated receiver module of FIGS. 1A to 1C is as follows. The DLI 2 is affixed to a housing 1 via a mount 7. The back-illuminated PIN-PD array 5 and the differential input TIA 6 are mounted on the top face of a carrier 9, and are electrically connected by wiring 13. In the carrier 9, a sloping face is formed on part of the carrier, and a mirror 4 is attached to the sloping face. The carrier 9 carrying the back-illuminated PIN-PD array 5 and the differential input TIA 6 is mounted onto the housing 1, and is locally sealed airtight by a lid 12. Glass 10 with an anti-reflection (AR) film is fixed in place by spacer glass 14 such that the face coated with the AR film is the face that emits optical beams from the AR-coated glass. The AR film of the AR-coated glass 10 prevents reductions in optical beam intensity due to reflections caused by the difference between the refractive indexes of glass and air when optical beams emitted from an output port and propagated inside the AR-coated glass are emitted from the glass. On part of the housing 1, there is formed a box shape housing components such as the back-illuminated PIN-PD array 5, the differential input TIA 6, and the carrier 9, with a window 11 attached to a wall surface thereof such that optical beams emitted from the DLI 2 are incident under the lid. Fiber 16 is fixed in place by a fiber block 15, and connected so as to be optically coupled with the DLI 2. Output ports 2a, 2b, 2c, and 2d of the DLI 2 are arranged on an edge face where the AR-coated glass 10 of the DLI 2 in FIG. 1B is applied, in the order 2a, 2b, 2c, and 2d from the top of FIG. 1B.
Light from the fiber 16 enters the DLI 2, and the four optical beams respectively emitted from the output ports 2a, 2b, 2c, and 2d of the DLI 2 are converted by lenses 3a and 3b into condensed beams which are transmitted through the window 11 and enter the interior of the locally airtight sealed package, and after their direction of propagation is converted upward by the mirror 4, are condensed respectively onto the photodetectors 5a, 5b, 5c, and 5d of the back-illuminated PIN-PD array 5, which is arranged such that the plane of optical incidence faces downward.
With the integrated receiver module of FIGS. 2A to 2C, the DLI 2 is affixed to the housing 1 via the mount 7, similarly to the integrated receiver module of FIGS. 1A to 1C. The back-illuminated PIN-PD array 5 is mounted on a side face of the carrier 9, while the differential input TIA 6 is mounted on the top face of the carrier 9, and are electrically connected by wiring 13. The carrier 9 carrying the back-illuminated PIN-PD array 5 and the differential input TIA 6 is mounted onto the housing 1, and is locally sealed airtight by a lid 12. The AR-coated glass 10 is fixed in place by spacer glass 14 such that the face coated with the AR film is the face that emits optical beams from the AR-coated glass. On part of the housing 1, there is formed a box shape housing components such as the back-illuminated PIN-PD array 5, the differential input TIA 6, and the carrier 9, with a window 11 attached to a wall surface thereof such that optical beams emitted from the DLI 2 is incident under the lid. Fiber 16 is fixed in place by a fiber block 15, and connected so as to be optically coupled with the DLI 2. Output ports 2a, 2b, 2c, and 2d of the DLI 2 are arranged on an edge face where the AR-coated glass 10 of the DLI 2 in FIG. 2B is applied, in the order 2a, 2b, 2c, and 2d from the top of FIG. 2B. An optical beam from the fiber 16 enters the DLI 2, and the four beams of light respectively emitted from the output ports 2a, 2b, 2c, and 2d of the DLI 2 are converted by lenses 3a and 3b into condensed beams which are transmitted through the window 11 and enter the interior of the locally airtight sealed package, and are condensed respectively onto the photodetectors 5a, 5b, 5c, and 5d of the back-illuminated PIN-PD array 5, which is arranged upright such that the plane of optical incidence faces in the direction of the DLI 2.
In these integrated receiver modules, the lenses 3a and 3b are aligned in the X, Y, and Z axis directions and affixed to the housing 1 after mounting the DLI 2 onto the housing 1 and mounting the carrier 9 carrying the back-illuminated PIN-PD array 5 onto the housing 1. In the case where error from the designed values occurs in the relative positional relationship of the DLI 2 and the back-illuminated PIN-PD array 5 due to factors such as mounting position misalignments produced when mounting the DLI 2 onto the housing 1, mounting the back-illuminated PIN-PD array 5 onto the carrier 9, and mounting the carrier 9 onto the housing 1, or dimensional tolerances of respective members, if the positional misalignment is horizontal misalignment in the X, Y, and Z directions, it is possible to condense optical beams emitted from the DLI 2 onto the photodetectors 5a, 5b, 5c, and 5d of the back-illuminated PIN-PD array 5 by adjusting the positions of the lenses 3a and 3b in the X, Y, and Z directions. With such an embodiment, it is possible to realize an integrated receiver module.